1. Technical Field of the Invention
The invention relates to an apparatus for determining the impedance (Zaw) of the respiratory tract by measuring the alternating pressure (dp) in the region of the mouth of a patient after producing an oscillating air pressure signal. The apparatus comprises of a mouthpiece, an electroacoustic transducer provided with a mechanical oscillation system for generating the oscillating air pressure signal, a tube for connecting the electroacoustic transducer to the mouthpiece, a reference resistance for determining the reference impedance (Zref), and a computing device for calculating the impedance (Zaw) of the respiratory tract on the basis of the reference impedance (Zref) of the reference resistance, the total impedance (Zges), and the total phase angle (φ).
The efficiency of a lung is determined by the effectiveness of gas exchange, that is to say in the delivery of oxygen to the blood and the removal of carbon dioxide from the blood. The two processes are dependent on a large number of parameters, such as on the lung volume, on the distribution of respiratory air within the lung, on the surface area of the alveolocapillary membranes, their thickness and diffusion properties, or on the ventilation. All these parameters, in the event of a deviation of the parameters from the norm, may form the causes of disturbances in lung activity.
Of particular medical interest, however, is the resistance of the respiratory tracts. An elevated respiratory tract resistance is one of the main indicators of an abnormal functioning of the lungs, as is demonstrated by, for example, asthmatics and allergies. With the determination of the “respiratory tract impedance”, the doctor has a, if not the, fundamental parameter of the lung function. All the known methods for determining the respiratory tract resistance, such as body plethysmography, flow/volume determination, occlusion pressure resistance, among other are indirect measurement methods that require not inconsiderable cooperation of the patient. Only determination of the respiratory tract resistances by means of the oscillatory method permits direct measurement of the respiratory tract resistance without the cooperation of the patient.
The term “impedance” is known from electrical technology and describes the current resistance in an alternating current circuit (alternating current resistance). In an electrical circuit with two poles, on which an alternating current is applied, the alternating current is displaced with respect to the applied current by a particular phase angle. The physically precise description of the phase displacement is possible by means of complex numbers. For a complex number z, in algebraic notation z=x+iy, in exponential notation z=r eiφ and in trigonometric notation z=r (cos φ+l sin φ). A complex number is given by the magnitude r=|Z| and the phase angle φ. In the case of respiratory impedance Zres, which represent a resistance as a complex parameter and is also known as total impedance (Zaw), model experiments have shown that Zres is also physically adequately defined by these two parameters.
The resistances of the respiratory tract can be termed in analogy to the resistances of an electrical circuit. The real resistance to respiration is always positive and is caused by the frictional losses in gas and tissue. The real respiratory resistance is also known as resistance (flow resistance) and corresponds to the real part (x) of the complex respiration resistance. The trachea and bronchia act as inductive resistances by virtue of the mass inertia of the gases they contain. Because of the compressibility of the air and because of the tissue elasticity, a capacitative resistance component also occurs. The total (in the sense of a vectorial addition) of the inductive and capacitative resistances is the reactance, which makes up the imaginary real part (y) of the complex resistance. The resistance and the reactance are given in [kPa/(l/s)]. The term resistance oscillation (Ros) corresponds to the respiratory impedance Zres at an oscillation frequency.
Since the computing equipment necessary for the analysis of complex numbers only became ready for production at a relatively late stage, the evaluation of the data determined as part of the oscillator resistance measurement at first only referred to the determination of respiration resistance as a real magnitude (German patent 1960 640), which is also termed the oscillatory resistance (Ros).
2. Description of the Prior Art
In the prior art, various methods are known for oscillatory determination of the respiratory tract resistance. Oscillations of 4 to 50 Hz are imposed on the patient's respiratory tract via the mouth with an open or closed nose.
The SIREGNOT FD5 appliance from Siemens superimposes a high frequency oscillation flow of about 10 Hz on the human respiratory frequency of 0.2 to 0.3 Hz. The superimposed oscillation current of 10 Hz is generated by a valveless diaphragm pump with a defined displacement. A plastic tube additionally connected to the mouthpiece with a known radius and a known length is oscillated parallel to the patient's respiratory tract. The tube serves as a reference resistance in the sense of an impedance, which is caused by the inductance of the oscillating air column. The patient can breathe unhindered through this reference tube, the volume of the tube only insignificantly enlarging the dead space. The oscillating volume flow generates an alternating pressure (dp) in the mouth, which is recorded by a microphone. The alternating pressure has a phase displacement (φ) with respect to the volume flow emerging from the generator of the air-pressure signal. After filtering, rectification and smoothing of the analogue voltage signal, the resistance is displayed on the instrument.
The value that can be read on the indicator instrument is calculated by means of a computing device, which is suitable for calculating the respiratory tract impedance (Zaw) from the reference impedance (Zref) and the total impedance (Zges) and the total phase angle (φ). The relationship between the total impedance (Zges) and respiratory tract impedance (Zaw) and the reference impedance (Zref) is governed by 1/Zges=1/Zaw+1/Zref, if Zaw and Zref are connected in parallel. By applying the calculating rules for complex numbers, the real part and imaginary part of the respiratory tract impedance (Zaw) can be determined. The reference impedance (Zref) is determined by the reference air tube, while the total impedance (Zges) is measured by a pressure tranducer, wherein it should be considered that the volume flow dV/dt remains constant. By virtue of this constancy, the measured alternating pressure at the mouth can be regarded as a measure of the total impedance (Zges). The computing device calculates the respiratory tract impedance (Zaw) and the real part of the respiratory tract impedance (Zaw) from the given reference impedance (Zref) and the given reference phase angle, and the measured alternating pressure at the mouth as a measure of the total impedance (Zges) and the measured total phase angle.
From the prior art, impulse oscillometry (IOS) is also known, in which an electrical square pulse is transformed by the mechanic properties of the downstream loudspeaker into a mixture of the desired frequencies. A lung function test device operating according to the principle of impulse oscillometry consists of a loudspeaker for generating the test pulses, a T-shaped connector, in which the modulation of the respiratory current is performed by the test signal. An opening of the T-connection leading away from the loudspeaker has a defined resistance with respect to the ambient air, via which the patient can breathe freely. The other T-piece opening leading away from the loudspeaker is connected to a pneumatograph, in which a pressure sensor is integrated. While the pressure sensor permits measurement of the mouth pressure (p), the volume flow rate (V) is measured by means fo the pneumatograph. The measurement results are read as electrical signals. The pressure-flow quotient is the thoracopulmonary impedance of the patient that is to be determined. For calibration of the measurement head, the reference impedance (Zref) is measured at the output of the pressure sensor that is connected downstream of the pneumonograph. For this purpose, a sieve resistance element, which can be designed with different shapes, is measured. German patent 432 63 74 A1 describes a conically flared reference impedance with sieve resistance, which is suitable for the calibration of the pressure and flow measuring equipment of a device for oscillometric measurement of the respiratory tract resistance.